PL/SQL newbie: Functions with SELECT and SYSDATE - sql

Question

I'm new to SQL and was hoping for some help turning this into a function:
SELECT
SYSDATE "Today's Date",
NEXT_DAY(trunc(SYSDATE, 'MONTH')-1, 'Tuesday')"First Tuesday this Month",
NEXT_DAY (LAST_DAY(SYSDATE)+1,'TUESDAY') "First Tuesday of Next Month"
FROM DUAL;
Here's my attempt at turning the above into a function (as you can see it didn't go so hot)...
CREATE OR REPLACE FUNCTION first_tuesday
RETURN DATE
AS
date1 DATE;
date2 DATE;
date3 DATE;
BEGIN
date1 := SELECT SYSDATE;
date2 := SELECT NEXT_DAY(trunc(SYSDATE, 'MONTH')-1, 'Tuesday');
date3 := SELECT NEXT_DAY (LAST_DAY(SYSDATE)+1,'TUESDAY');
RETURN 'todays date:' || date1,
'first tuesday this month:' || date2,
'first tuesday next month:' || date3;
END;
/
SELECT first_tuesday FROM DUAL;
What am I doing wrong?

Answer 1

You're really trying to return three values there. I'd suggest breaking it out into separate functions, but leave "today's date" just as a call to sysdate.
You don't need to issue a SELECT query to populate a variable, and in this case you can just return an expression.
CREATE OR REPLACE FUNCTION first_tuesday
RETURN DATE
AS
BEGIN
RETURN NEXT_DAY(trunc(SYSDATE, 'MONTH')-1, 'Tuesday')
END;
/
Something to look out for here is that within the execution of a SQL statement, SYSDATE always has the same value, but that's not the case in PL/SQL. So when sysdate is evaluated in PL/SQL that is called from a SQL statement, you can get a different value in each context. So if you call:
select sysdate col1,
some_function col2
from dual;
... and during it's lengthy execution some_function calculates sysdate, it could be using a slightly different (later) value than the parent SQL statement.
The way to avoid this is to pass sysdate into the function:
CREATE OR REPLACE FUNCTION first_tuesday (sysdate_in date)
RETURN DATE
AS
BEGIN
RETURN NEXT_DAY(trunc(sysdate_in, 'MONTH')-1, 'Tuesday')
END;
/
So then you might ...
Select ...
From invoices
Where invoice_date < first_tuesday(sysdate)
That's a trivial example just to demonstrate the principle, and it really only matters for functions called from SQL.
To be honest, most practitioners in your situation would probably skip the function and just:
Select ...
From invoices
Where invoice_date < NEXT_DAY(trunc(sysdate, 'MONTH')-1, 'Tuesday')
Edit: by the way, you probably want to use:
NEXT_DAY (LAST_DAY(SYSDATE),'TUESDAY')
... not ...
NEXT_DAY (LAST_DAY(SYSDATE)+1,'TUESDAY')